liblp: refactor MetadataBuilder::NewForUpdate

Move the logic for retrofit DAP to its new function. The new flow
in NewForUpdate is:
    metadata = ReadMetadata();
    if (retrofit dap) UpdateMetadataForOtherSuper(metadata)
    return metadata;

Test: liblp_test_static
Change-Id: I6890fff3a7c44ebe2004de96b2ccbe1e8ce37546